Sex Linked TraitsSex Linked Traits

J. R. JonesJ. R. Jones

Spring 2008Spring 2008

Human ChromosomesHuman Chromosomes Chromosomes are only visible during cell Chromosomes are only visible during cell

division (mitosis/meiosis).division (mitosis/meiosis). To analyze chromosomes, biologist To analyze chromosomes, biologist

photograph cells during mitosis and align photograph cells during mitosis and align the chromosomes in homologous pairs.the chromosomes in homologous pairs.

This picture is known as a karyotype.This picture is known as a karyotype.

http://www.miscarriage.com.au/images/pages/karyotype_normal.jpghttp://www.biotechnologyonline.gov.au/images/contentpages/karyotype.jpg

On a human karyotype:On a human karyotype: Chromosome pairs 1-22 are known as autosomes.Chromosome pairs 1-22 are known as autosomes.

They code for traits that are not related to sex.They code for traits that are not related to sex. Both males and females have the same types of Both males and females have the same types of

autosomes.autosomes.

Chromosome pair 23 includes the sex Chromosome pair 23 includes the sex chromosomes.chromosomes.

They determine the biological sex traits of an individual.They determine the biological sex traits of an individual. Males have XY and females have XX.Males have XY and females have XX.

Types of ChromosomesTypes of Chromosomes

Punnett Squares with Sex Punnett Squares with Sex ChromosomesChromosomes

Remember: Sex is Remember: Sex is determined by the determined by the father of a child.father of a child.

Every time a Every time a couple has a baby, couple has a baby, there is a 50% there is a 50% chance of having a chance of having a boy or a girl.boy or a girl.

XX XY

XX XY

X Y

X

X

Sex-linked TraitsSex-linked Traits Sex chromosomes can also Sex chromosomes can also

determine traits that do not effect determine traits that do not effect your biological sex.your biological sex.

Only the X chromosome carries Only the X chromosome carries alleles for sex-linked traits.alleles for sex-linked traits. These alleles are written as superscripts.These alleles are written as superscripts.

Remember Dragon Meiosis – XRemember Dragon Meiosis – XHH for horns, X for horns, Xhh for no horns.for no horns.

The Y chromosome does not carry The Y chromosome does not carry any alleles for sex-linked traits.any alleles for sex-linked traits.

X ChromosomeX Chromosome

Y ChromosomeY Chromosome

Thomas Morgan’s Thomas Morgan’s ExperimentExperiment

One of the first scientists to One of the first scientists to investigate sex-linked traits was investigate sex-linked traits was Thomas Morgan (1866-1945).Thomas Morgan (1866-1945).

Morgan was an embryologist by Morgan was an embryologist by trade, and wanted to determine trade, and wanted to determine what factors influenced human what factors influenced human inheritance.inheritance. He conducted experiments to He conducted experiments to

observe the inheritance patterns of observe the inheritance patterns of eye color in eye color in DrosophilaDrosophila (fruit flies). (fruit flies).

There are three possible eye colors There are three possible eye colors in fruit flies – red, sepia and white.in fruit flies – red, sepia and white.

Red is dominant to sepia both are Red is dominant to sepia both are dominant to white.dominant to white.

http://www.cas.vanderbilt.edu/bsci111b/drosophila/flies-eyes-phenotypes.jpg

Morgan’s ExperimentMorgan’s Experiment Consider the following crosses that Morgan conducted: R – red Consider the following crosses that Morgan conducted: R – red

eyes r – white eyeseyes r – white eyes Red eyed male x heterozygous red eyed femaleRed eyed male x heterozygous red eyed female

What are the genotypes of the parents?What are the genotypes of the parents? XXRRY, XY, XRRXXrr

Show the cross between these two parents, giving the phenotype and genotype Show the cross between these two parents, giving the phenotype and genotype ratios.ratios.

White eyed male x homozygous red eyed femaleWhite eyed male x homozygous red eyed female What are the genotypes of the parent?What are the genotypes of the parent?

XXrrY, XY, XRRXXRR

Show the cross between these two parents, giving the phenotype and genotype Show the cross between these two parents, giving the phenotype and genotype ratios.ratios.

What sex are the white eyed offspring?What sex are the white eyed offspring? All would be males.All would be males.

Could a male be a carrier for white eye color?Could a male be a carrier for white eye color? No, his one allele for recessive eye color will be expressed.No, his one allele for recessive eye color will be expressed.

Is it possible for a female fly to have white eyes?Is it possible for a female fly to have white eyes? Yes, but both of her parents would have to be white-eyed.Yes, but both of her parents would have to be white-eyed.

Morgan’s ConclusionsMorgan’s Conclusions The inheritance of certain traits is The inheritance of certain traits is

linked to biological sex. linked to biological sex. Morgan noticed that a large majority Morgan noticed that a large majority

of the white-eye flies were male.of the white-eye flies were male. Males are more likely to show a Males are more likely to show a

sex-linked trait because they have sex-linked trait because they have only one X chromosome. only one X chromosome. If they inherit a recessive allele X If they inherit a recessive allele X

chromosome, there would be no chromosome, there would be no dominant allele present to mask it.dominant allele present to mask it.

A carrier has the recessive allele in A carrier has the recessive allele in it’s genotype, but this is not it’s genotype, but this is not expressed in their phenotype. expressed in their phenotype. Males cannot be carriers of sex-linked Males cannot be carriers of sex-linked

traits.traits.http://www7.nationalacademies.org/archives/4x6_ThomasHuntMorgan.jpg

Sex-linked Traits in HumansSex-linked Traits in Humans Morgan’s conclusions explained the Morgan’s conclusions explained the

high incidence of certain high incidence of certain traits/diseases within predominantly traits/diseases within predominantly male populations.male populations.

Color blindness is a commonly known Color blindness is a commonly known sex-linked trait in humans.sex-linked trait in humans.

Are any of you colorblind?Are any of you colorblind?

When Harry Met Sally…When Harry Met Sally… Harry and Sally are two normal vision adults who have Harry and Sally are two normal vision adults who have

a color blind son.a color blind son. B – normal vision b – color blindB – normal vision b – color blind Show the cross between Harry and Sally.Show the cross between Harry and Sally. What are the genotypes of Harry and Sally?What are the genotypes of Harry and Sally?

XXBBY, XY, XBBXXbb

Give all of the possible genotypes and phenotypes of their Give all of the possible genotypes and phenotypes of their offspring.offspring.

XXBBXXB B – normal vision daughter– normal vision daughter XXBBXXb b – normal vision daughter (carrier)– normal vision daughter (carrier) XXBBY – normal vision sonY – normal vision son XXbbY – color blind sonY – color blind son

Sally’s dad has normal vision. Is Sally’s Sally’s dad has normal vision. Is Sally’s Mom colorblind? Mom colorblind?

No, but she is a carrier.No, but she is a carrier.

http://www1.istockphoto.com/file_thumbview_approve/2341727/2/istockphoto_2341727_cute_cartoon_couple_drinking_vector.jpg

Other sex-linked traits:Other sex-linked traits:

Baldness (some types)Baldness (some types) Muscular DystrophyMuscular Dystrophy HemophiliaHemophilia

All of these traits happen to be sex-All of these traits happen to be sex-linked recessive.linked recessive.